Bluetooth: Move hci_alloc/free_dev close to hci_register/unregister_dev

alloc() and register() (and free() and unregister()) are closely related
so move them more closely together. This will also allow to move
functionality from register() to alloc() without needing
forward-declarations.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>